1. Field of the Invention
The present invention relates to a color sensor using a diffraction grating for color recognition or colorimetry on a subject and, more particularly, is suitable for a color sensor which is incorporated in a color image forming apparatus to perform colorimetry on toner or printing media.
2. Description of the Related Art
In an image forming apparatus for forming a color image through an electrophotographic process, a deviation in color tone may occur because of color mixing of toner. The same problem occurs, not only in the electrophotographic type image forming apparatus but also in general image forming apparatuses for forming a color image such as an ink-jet type image forming apparatus. In the electrophotographic type, particularly, drum sensitivity, the charge capacity of toner, and transfer efficiency to a paper sheet are changed for each color depending on the condition of environment, and hence a color mixing ratio is deviated from a predetermined value and thus it is highly likely to affect a color tone. Further, there is a need for the realization of higher-degree of color tone reproduction in the print industry.
In order to solve the problems described above, Japanese Patent Application Laid-Open No. H09-160343 proposes a method of measuring a spectral reflection light quantity of a toner image using two different spectral filters and correcting an image signal based on the result of the measurement. Similarly, Japanese Patent Application Laid-Open No. 2004-126278 proposes an image forming apparatus for performing color tint correction on a color image. In the electrophotographic type image forming apparatus, according to Japanese Patent Application Laid-Open No. 2004-126278, a color tint detection sensor is provided in the downstream of a fixing device located on a fixing and conveying path to detect RGB output values of colors of a mixed color patch image formed on a transferring material (printed image) moving along a conveying path.
In order to determine the color tone at higher precision, it is necessary to increase the number of spectral wavelength bands to at least three, which corresponds to the number of primary colors. When the number of wavelength bands may be further increased, the color tone may be determined at higher precision. In order to increase the number of wavelength bands, there have been proposed a large number of diffraction spectral devices for performing spectral measurement based on a diffraction phenomenon, for example, in Japanese Patent Application Laid-Open No. H06-058812, Japanese Patent Application Laid-Open No. H06-050814 and Japanese Patent Application Laid-Open No. 2001-264173.
An example of a colorimetric device capable of measuring absolute chromaticity at high wavelength resolution is a spectral colorimetric apparatus for obtaining chromaticity from a spectral intensity distribution using a diffraction spectroscope.
FIG. 9 illustrates a Rowland type diffraction spectral colorimetric apparatus generally used as a conventional diffraction spectral device.
A light beam to be detected is entered into a detection optical apparatus from an incident window 108. Specifically, scattering light from a subject illuminated by an illumination optical system (not shown) is entered into the detection optical apparatus from a stop. The light is spectrally separated by a concave reflective diffraction element 104 and then obtained as a spectral intensity distribution by a one-dimensional array detector 103.
In a color image forming apparatus for forming a color image as illustrated in FIG. 8, the spectral colorimetric apparatus is required to be incorporated for use into the main body of the color image forming apparatus. In practice, a reduction in size of the spectral colorimetric apparatus in a conveying direction of a printed paper sheet, that is, in a direction perpendicular to a detection surface is strongly demanded so as not to hinder the conveying of the printed paper sheet in the color image forming apparatus.
The image forming apparatuses disclosed in Japanese Patent Application Laid-Open No. H09-160343 and Japanese Patent Application Laid-Open No. 2004-126278 employ a colorimetric device using an RGB filter. The colorimetric device is small in size but the number of wavelength bands is small, and thus is not suitable to measure accurate absolute chromaticity. Further, when the number of filters is to be increased, there is a problem that a significant increase in cost occurs.
In the case of a Rowland type spectral device, which is generally used as the diffraction spectral device, imaging magnification effected by the diffraction grating shows substantially qui-magnification, that is, the ratio of the size of the stop and the image thereof on the array detector are substantially 1. When a Rowland circle is made smaller, the spectral unit may be relatively easily reduced in size. However, when the entire structure including the illumination system is taken into account, there is still room for improvement. In the device disclosed in Japanese Patent Application Laid-Open No. H06-058812, the detection surface and the spectral plane are perpendicular to each other, and hence the device is increased in size in a direction perpendicular to the detection surface. In the device disclosed in Japanese Patent Application Laid-Open No. H06-050814, a test subject and a stop are made conjugate with each other so that a light beam to be detected which is emitted from the test subject forms an image on a stop through a mirror and an imaging lens. The optical path of the light beam to be tested is folded by the mirror to reduce the device in size. However, the detection surface and the spectral plane are made perpendicular to each other, and hence the device is increased in size in the direction perpendicular to the detection surface.
There is another problem that, when the entire apparatus including the illumination system is reduced in size, it is difficult to remove flare light from the illumination system. When the flare light reaches a sensor, a noise is superimposed on an output of the sensor, making it difficult to perform accurate chromaticity measurement.